In this post halogen lamp draws very high current at switch ON.
Cold filament resistance is very low and this causes sudden inrush current.
Lamp life becomes short with this circuit protects lamp and it also protects outside fuse and switch indirectly.
Soft ON Circuit for 12V Halogen Lamp reduces this problem.
Lamp voltage rises slowly which turns ON smoothly and its life increases.
Circuit Working:
Parts List:
| Components | Specification | Quantity |
|---|---|---|
| Resistors (All resistors are 1/4 watt unless specified) | 10k | 2 |
| 100k, 390Ω, 2.7Ω 5 watt | 1 each | |
| Capacitors | Electrolytic 220uF 25V | 2 |
| Semiconductors | Transistor BC547 | 2 |
| Optocoupler / Opto-Triac S202DS4 | 1 | |
| Bridge Diode 1N4148 | 4 | |
| 12V 50W Halogen Bulb | 1 |
12V AC 50W supply is given to the circuit.
Bridge rectifier changes AC to DC with capacitors smooth the DC voltage.
C1 and C2 charge slowly and this creates time delay.
At power ON C2 is empty with Q1 transistor stays OFF and Q2 transistor is also OFF.
Opto-triac does not conduct and lamp gets very low voltage.
With time C2 charges through R1 and R2 and base voltage of Q1 increases slowly.
Q1 turns ON slowly and Q2 also turns ON.
Current flows through R4 to opto-triac LED and trough this Opto-triac triggers slowly.
Triac conduction angle increases with lamp voltage rises smoothly.
After few seconds the lamp reaches full brightness with inrush current is limited and lamp filament is protected.
Formulas:
Formula for rectified DC voltage:
Vdc = Vac x 1.414 – diode drop
where,
- Vdc means DC voltage after rectifier.
- Vac means AC RMS input voltage.
- 1.414 is square root of 2 value.
- Diode drop means voltage loss in rectifier diodes.
How to Build:
To build a Soft ON Circuit for 12V Halogen Lamp follow the below steps:
- Take all the parts as shown in circuit diagram.
- Bridge Rectifier positive pin goes to C1 and C2 positive.
- negative pin goes to ground line.
- Rest two pins connect to 12V 50 watt
- BC547 Q1 emitter pin connect to ground.
- Base pin connect to R2 and C2 junction.
- Collector pin connect to junction of base of T2 and resistor R3
- T2 BC547 emitter pin goes to ground.
- Base connect from Q1 collector and R3.
- Collector goes to R4 and opto-triac LED.
- Opto-Triac S202S04 input LED pins connect to R4 and T2 collector.
- Output triac pins connect in series with lamp.
- Lamp connected in AC line through opto-triac.
- Resistor R5 is connected parralel to Opto-Triac
Conclusion:
This Soft ON Circuit for 12V Halogen Lamp is simple with
Parts are easily available and it protects halogen lamps.
Lamp life increases with inrush current is reduced.
Best for 12V halogen lighting systems.